I am looking for a cloud platform that will sync my PDF library on my Android phone and Win10 laptop and tablet.

For instance, if I download or create a new PDF on my phone and save it on the SD Card, I'd like it to go to the cloud and then be automatically inserted into the libraries on my laptop and MS Surface Go SD Card.

The plan is to subscribe to the service. Most have a 1 or 2 TB service for $10/month, which is perfectly OK. I have Dropbox, Mediafire, Google Drive and OneDrive with different small shared items on each. The only one I pay for is Mediafire. But, I'm looking to commit to one for my files. Research has given me some good leads, but none really talks about two-way synchronization.